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SPE 75158 AN ALTERNATIVE MODEL FOR FILTER CAKE FORMATION IN FRACTURES Randy Seright

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SPE 75158 AN ALTERNATIVE MODEL FOR FILTER CAKE FORMATION IN FRACTURES Randy Seright New Mexico Petroleum Recovery Research Center. TOPICS COVERED IN SPE 75158 New model for filter cake formation in fractures.

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slide1

SPE 75158

AN ALTERNATIVE MODEL FOR FILTER CAKE FORMATION IN FRACTURES

Randy Seright

New Mexico Petroleum Recovery Research Center

slide2

TOPICS COVERED IN SPE 75158

New model for filter cake formation in fractures.

Effects of temperature and gel composition on extrusion for Cr(III)-acetate-HPAM gels.

Extrusion behavior for a guar-borate gel.

Effects of gel composition, fracture width, and addition of particulates on washout of gels from fractures (during brine injection).

slide3

WHAT ARE GEL PROPERTIES DURING EXTRUSION THROUGH FRACTURES?

  • GEL USED:Cr(III)-acetate-HPAM
  • 0.5% Ciba Alcoflood 935 HPAM,

0.0417% Cr(III)-acetate,

1% NaCl, 0.1% CaCl2, pH=6.

  • Gelation time ~ 5 hrs at 41°C.
  • Aged 24 hours at 41°C before use.
  • Injected into brine-saturated fractured

cores,41°C.

slide4

PROPERTIES OF Cr(III)-ACETATE-HPAM

  • GEL DURING EXTRUSION THROUGH FRACTURES
  • A minimum pressure gradient must be met before a formed gel will extrude through a fracture.
  • Once the minimum pressure gradient is met, the pressure gradient during gel extrusion is not sensitive to injection rate.
  • Once the fracture is filled with gel, pressure gradients are stable (no progressive plugging).
  • The pressure gradient for gel extrusion varies inversely with the square of fracture width.
slide5

PROPERTIES OF Cr(III)-ACETATE-HPAM

  • GEL DURING EXTRUSION THROUGH FRACTURES
  • Gels dehydrate, thus retarding the rate of movement of the gel front.
  • Although water leaks off through the fracture faces, crosslinked polymer cannot.
  • Dehydrated (concentrated) gel is immobile.
  • Mobile gel is the same as the injected gel.
  • Mobile gel wormholes through immobile gel.
slide6

Leakoff rate = total matrix production rate

total fracture area

wf = 0.04 in.

Average Injection Flux

ul = 0.05 t -0.5

slide7

WHY DOES ul= 0.05 t -0.5 ?

Analogy with leakoff from hydraulic fracturing:

ul = C1 t-0.5 from SPE Monograph Volume 2 (1970), p. 33.

Basis: Assume formation of a filter cake of uniform thickness, L, and permeability, k, with pressure drop, p.

ul= p k / ( L) and L = C2  ul dt

Solution:ul = C1t-0.5C1 = [pk/(C2)]0.5

fluid flow in fracture

filter cake, k

L

p

water leakoff

fracture face

porous rock

slide8

WHY DOES

  • ul= 0.05 t-0.5 ?
  • Our problem is slightly different:
  • The filter cake is not areally uniform.
  • Fresh gel wormholes through concentrated gel.
slide9

BASIS OF NEW MODEL (Model details are in SPE 75158):

On first contact, fracture area is covered with mobile gel.

With time, more and more concentrated gel accumulates.

Area contacted by mobile gel decreases with time.

Mobile gel forms wormholes through immobile gel.

Dominant leakoff is from mobile (wormhole) gel.

Fraction of fracture area covered by immobile concentrated gel

30 minutes

Fraction of fracture area covered by mobile wormhole gel

1 minute

slide10

CONVENTIONAL FILTRATION MODEL

  • VERSUS THE NEW MODEL (SPE 75158)
  • CONVENTIONAL MODEL ASSUMES:
  • Areally uniform coverage with filter cake.
  • Leakoff rate decreases with time because the filter cake becomes thicker with time.
  • NEW MODEL ASSUMES:
  • Two types of areal coverage
    • Mobile gel—the dominant water source.
    • Immobile gel—minor water source.
  • Leakoff rate decreases with time because the area covered by mobile gel decreases with time.
slide11

Summary of all tests to date at 41°C.

Gel: 0.5% HPAM, 0.0417% Cr(III)-acetate, 41°C.

Flux: 129 to 66,200 ft/d,

Lf : 0.5 to 32 ft,

hf : 1.5 to 12 inches,

wf : 0.02 to 0.16 inches.

wf,

inches

New model

ul = um / [1 + ul dt /wf]

ul = 0.05 t -0.5

slide12

Conventional

filter cake model

Vertical views

intersecting

the fractures

New model

Active flow path

Filter cake or

concentrated gel

Fracture height

Porous rock

Fracture width

{

{

slide13

Pressure gradients required to extrude a Cr(III)-acetate-HPAM gel through fractures

dp/dl=0.02/wf2

slide14

Conventional

filter cake model

Vertical views

intersecting

the fractures

New model

Active flow path

Filter cake or

concentrated gel

Fracture height

Porous rock

Fracture width

{

{

slide15

Gel injection rate = 200 cm3/hr or 413 ft/d,

Lf x hf x wf = 48x1.5x0.04-in.

Pressure Behavior in the Fracture During Gel Injection

After 1st gel contact, theconventional model predicts that flow channel width should decrease and pressure gradient should increase with gel throughput. The new model predicts that they should remain about the same.

slide16

CONCLUSIONS

For extrusion of Cr(III)-acetate-HPAM gels through fractures in water shutoff applications, a new filter cake model was developed that:

is based on fracture area in contact with immobile concentrated gel increasing with time rather than filter cake thickness increasing with time.

predicts leakoff rates similar to those for the conventional filter cake model.

correctly predicts that pressure gradients during gel extrusion do not increase with throughput.

may be of interest in other areas (i.e., hydraulic fracturing, produced water re-injection).

slide17

LEAKOFF RATES:

Guar-borate vs. Cr(III)-acetate-HPAM

Average dp/dl = 21.7 psi/ft,

Breakthrough at 9.2 fracture volumes

New model for

Cr(III)-acetate-HPAM

0.36% guar, 0.018% NaBO2,

0.24% tallow soap, 0.1% surfactant,

aged 1 day at 40°C,

Lfx hfx wf= 6x1.5x0.04 in.,

Injection flux = 4,130 ft/d.

slide18

0.36% guar, 0.018% NaBO2, 0.24% tallow soap, 0.1% surfactant,

aged 1 day at 40°C, Lfx hfx wf= 12x12x0.04 in.

Flow direction

slide19

Compared to the conventional filter cake model, if dp/dl and the total filter cake volume are fixed, the new leakoff model predicts:

  • similar leakoff rate behavior,
  • higher average fluid velocities,
  • higher shear rates at the fracture wall,
  • higher shear stresses at the fracture wall.
slide20

PROPERTY COMPARISONS FOR A POWER-LAW FLUID:

New Model Prediction Divided by Conventional Model Prediction

wc= flow opening for conventional filter cake model

ww= wormhole width ww/ wc > 1 -1 n 0

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